PCS Wales Secretary, Shavanah Taj said: “This is a big step, which we’ve not taken lightly, but Museum management have upped the ante by refusing even to talk about improving their compensation offer and by bullying staff into signing away their rights to weekend pay.

“We have been prepared to negotiate all along and that’s still what we want.

“Since we made this decision, we’ve already had an offer from the Museum to talk to them about reducing the number of weekends that their staff have to work.

“They’ve previously refused to discuss this, so this is already a step forward, in response to the pressure that we’ve applied. We now need to see some movement on financial compensation.”

National Museum Wales director general David Anderson has previously said that the service can no longer afford to pay £70,000 a month in premium payments.

Campaigners near the Cardiff City Stadium in protest over museum pay row

He said the Welsh Government had made it clear to him and to PCS that it has no more money to give to the museum network.

Mr Anderson has blamed the PCS for the breakdown of talks, claiming the union had reneged on a deal brokered by the government conciliation service Acas that would have seen workers affected by the cut getting a 4% pay rise plus an average one-off lump sum payment of £4,000.